In prior art, polymer-based electrical insulation, e.g. extruded solid insulation based on a polyethylene, or a crosslinked polyethylene (XLPE), is often used for alternating current (AC) and direct current (DC) transmission and distribution cable insulation. To ensure that the electric power cables have good or correct dielectric properties and e.g. that any voids in the polymer-based electrical insulation are free of gas, the polymer-based electrical insulation is exposed to a heat treatment procedure after the electric power cable has been produced or assembled. For example, by means of the heat treatment procedure, the concentration of gaseous by-products, e.g. methane, acetophenone or cumyl alcohol, in the polymer-based electrical insulation is reduced or diminished. The by-products may originate from the crosslinking reactions. Acetophenone or cumyl alcohol by-products enhance the electrical conductivity. In prior art it is known to place the electric power cable having a polymer-based electrical insulation in a heating chamber in which the electric power cable is exposed to the heat treatment procedure. Often the electric power cable is first wound around a cable drum and then the cable drum with the electric power cable is placed in the heating chamber. This procedure may be called cable drum heat treatment.
US 2010/0163273-A1 discloses a process for preparing a crosslinked cable. One or more layers including a polymer composition is/are applied on a conductor, wherein at least one layer includes one or more free radical generating agents. Said at least one layer including said free radical generating agent(s) is crosslinked by radical reaction. The content of volatile decomposition products(s) is reduced or removed. It disclosed that the cable may be wound onto a cable drum and thereafter placed into a ventilated heating chamber in order to expose the cable to degassing at an elevated temperature.
US2010/0314022-A1 discloses a method for providing an insulated electric high voltage DC cable comprises the steps of providing a polymer-based insulation system comprising a compounded polymer composition; and subsequently exposing the polymer-based insulation system to a heat treatment procedure while the outer surface of the polymer-based insulation system is covered by a cover impermeable to a substance present in the polymer-based insulation system in a non-homogenous distribution.
JP2002-260464 describes a method for removing crosslinked residue of a polyolefin insulating cable. The cable wound around a cable drum is stored in a container and an electric current is passed to the cable while reducing the pressure in the container.
JP11-185553 discloses a cable heating room and a cable drum transfer device 5 including a carrying belt provided therein.
WO 01/72493-A1 discloses a method for removing by-products produced in a cable vulcanization process. In the method circulation gas is led from a gas space of vulcanization equipment into a gas circulation means after which the gas is led into a condenser where it is cooled, the by-products are condensed into a purification cell comprising a condensation surface, and the substantially purified circulation gas is reintroduced into the gas space of the vulcanization equipment.
The article “The Role of Degassing in XLPE Power Cable Manufacture” by T. Andrews et. al., Electrical Insulation Magazine, IEEE, vol. 22, No. 6, pp. 5-16, November-December 2006 (ISSN: 0883-7554), discloses the degassing of large power cables in large, heated chambers, the chambers being ventilated to avoid build-up of flammable methane and ethane.
The article “A New Method for the Detection and Quantification of Residual Volatiles in XLPE Electrical Cable Using Large-Spot Raman Spectroscopy” by Mark S. Kemper et. al., IEEE Transactions on Power Delivery, vol. 26, No. 1, pp. 3-10, January 2011 (ISSN: 0885-8977), discloses a section of a cable being placed in a degassing oven maintained at about 80 degrees at reduced pressure.